DNA mismatch repair (MMR) is a conserved repair pathway and is essential for maintaining genomic integrity in prokaryotes and eukaryotes. MMR has been the focus of intensive research efforts because mutations in MMR genes are the underlying cause of Lynch syndrome (HNPCC, hereditary nonpolyposis colorectal cancer) and a significant proportion of sporadic colorectal cancers. The majority of these cancer cases are caused by mutations in the human homologs of the E. Coli mutS and mutL genes. The human MutS and MutL proteins form heterodimeric complexes that mediate the initial steps of MMR, including the recognition of mismatched base(s) arising from errors in replication, and signaling downstream proteins to facilitate mismatch removal. However, MMR complexes also recognize damaged-base mispairs resulting from exposure to environmental genotoxins or treatment with chemotherapeutic agents and mediate cell cycle arrest and apoptosis. As a consequence of their defective MMR, Lynch syndrome tumors and sporadic colorectal cancers display increased instability at short repeat sequences, termed microsatellite instability. In addition, these tumors display resistance to DNA damaging agents, and thus fail to respond to conventional chemotherapy. For example, while the 5-Fluorouracil (5-FU) treatment of MMR-proficient colorectal cancers results in a beneficial outcome, the same is not the case with MMR-deficient sporadic colorectal or Lynch syndrome cancers. Of additional concern, the treatment of cancer patients with conventional chemotherapeutic agents frequently induces secondary therapy-related leukemias (e.g. Acute Myeloid Leukemia / Myelodysplastic Syndrome), which may be caused by selection for hematopoietic precursor cells with MMR-defects. As these cells proliferate they accumulate further mutations and increased resistance to anticancer agents. Thus the development of novel therapeutic strategies that efficiently and selectively target primary cancers and prevent the formation of therapy-induced secondary cancers would be highly desirable. A promising new direction to achieve these goals is the harnessing of synthetic lethality, where the simultaneous loss of two otherwise non- essential factors is fatal for cells. In this application we propose to identify the core genetic networks related to MMR in two highly divergent model eukaryotes, the fission (Schizosaccharomyces pombe; Sp) and budding (Saccharomyces cerevisiae; Sc) yeasts. We will utilize this information to predict (and test) synthetic sick or lethal interactions in defined MMR- deficient mouse and human cancer cell lines. Our proposed studies will not only identify novel interactions and/or functions of the MMR genes, but also have the potential to identify highly effective chemotherapeutic strategies for a prevalent human cancer syndrome. PUBLIC HEALTH RELEVANCE: The DNA mismatch repair system (MMR) is essential for maintaining the integrity of mammalian genomes by correcting mismatched base pairs that result from erroneous replication or environmental damage. Defects in MMR are associated with a significant proportion of sporadic colorectal cancer, and are the underlying cause of the Lynch cancer syndrome (also known as HNPCC: hereditary nonpolyposis colorectal cancer). We are studying yeast and mammalian cell lines with mutations in key MMR genes to identify key conserved genetic networks for MMR. The goal of our studies is not only to determine novel interactions and/or functions of essential MMR genes, but also to identify highly effective chemotherapeutic strategies for a prevalent human cancer syndrome.